Amnesty International says the “botched” execution in Oklahoma on April 30 provides another stark reason why U.S. authorities should impose a moratorium on judicial killing and work to abolish the death penalty.

Witnesses have said that Clayton Lockett began to gasp and writhe after the first drugs were administered. About 16 minutes after the lethal injection process began, officials drew a curtain across the viewing window, preventing witnesses from seeing what was happening. Almost half an hour later, Lockett was pronounced dead of a heart attack.

A second execution scheduled for the same evening, of Charles Warner, was stayed.

“What happened … to Clayton Lockett is shocking in anyone’s book. But this is far from the first 'botched execution' in the USA, whether by electrocution, asphyxiation, or lethal injection using the ‘traditional’ three-drug protocol,” said Rob Freer, Amnesty International researcher on the USA. He cited more than three dozen executions reported to have gone awry.

The sole U.S. manufacturer of sodium thiopental, one of the drugs traditionally used in U.S. lethal injections, withdrew from the market in early 2011 and the European Commission tightened its regulations on the trade of such substances for use in capital punishment. As a result, the nation's death penalty states have sought alternative sources for lethal injections drugs and have amended their execution protocols.

“If the sort of tenacity shown by authorities pursuing the death penalty were to be turned to bringing their country into line with the global abolitionist trend, then we would see rapid progress on this fundamental human rights issue in the USA,” Freer said. “Instead, the ugly history of US executions has continued well into the 21st century even as country after country has stopped this practice.”

Lockett and Warner had unsuccessfully challenged an Oklahoma state law that blocks officials from revealing the identities of those involved in administering executions as well as of those who supply the drugs or equipment used.

Lockett, 38, was convicted of killing 19-year-old Stephanie Neiman in 1999. She was shot and buried alive. Lockett also was convicted of raping Neiman's friend in the home invasion.

Warner, 46, was convicted of raping and killing 11-month-old Adrianna Waller in 1997. He lived with the child's mother.